Lithium ion secondary batteries are being employed as main power sources of small portable electronic appliances because they have remarkable advantages including that they show a high energy density and are hardly degraded after a number of charging/discharging cycles if compared with other secondary batteries.
Known lithium ion secondary batteries include those formed by putting a battery element into a metal can and sealing the can and those formed by covering a battery element by a film casing to produce a battery case and sealing the case.
Particularly, there is a demand for large-capacity batteries in power supply applications such as electric automobiles, hybrid automobiles and various codeless machines such as codeless electric tools. Stacked batteries formed by casing a battery element by a film casing to produce a battery case and sealing the case are believed to be particularly suitable for such applications because of a high weight efficiency and a high volume efficiency they provide.
Such a film casing is prepared by laying a film such as a polyethylene film that is highly resistive against corrosion in electrolytic solutions and showing an excellent heat sealing effect and a highly corrosion-resistant film such as a polyimide film or a polyester film respectively on the inner surface and on the outer surface of an aluminum foil.
FIGS. 10A, 10B and 10C of the accompanying drawings schematically illustrate an example of a stacked battery sealed by a film casing. FIG. 10A is a schematic perspective view of the stacked battery and FIG. 10B is a schematic plan view of the stacked battery, while FIG. 10C is a schematic cross sectional view of the stacked battery taken along line A-A′ in FIG. 10B.
The stacked battery 1 is sealed by a film casing. It has a positive terminal 9 and a negative terminal 10 that operate as external connection terminals and are drawn out from the heat seal section of the film casing 2 by way of easily adhering seal members 11 that respectively cover the positive terminal 9 and the negative terminal 10 in order to prevent any liquid leakage from around the external connection terminals.
A flat stacked body 15 is formed by laying positive electrodes 6 and negative electrodes 8, both of which are flat and plate-like, one on the other by way of separators 7, and sealed by a film casing 2 and the prepared case is subsequently sealed to produce a stacked battery. Therefore, the stacked body can move in the case as the stacked battery is subjected to vibrations in any of the three directions that are orthogonal relative to each other. Then, as the stacked body 15 moves in the film casing 2 due to vibrations and repeatedly rubs the inner surface of the film casing 2, holes and fissures can be produced at the four corners of the film casing 2 and the parts thereof where the electrode terminals are drawn out to give rise to liquid leaking sections 20 as shown in FIG. 10B.
Proposals have been made to arrange a hygroscopic agent in parallel with the exposed surfaces of the stack of a battery element to absorb moisture in the inside of the film casing and alleviate the vibrations applied to the battery in a sealed battery in which a stacked body is sealed by a film casing (refer to, e.g., JP-A-2005-056672).
However, the hygroscopic agent arranged in parallel with the exposed surfaces of the stacked body of a battery element cannot prevent the film casing from being damaged when the stacked body is subjected to large vibrations for a long period of time.
However, while such a technique of providing aligning sections can hold the positional integrity of the positive electrodes and the negative electrodes of the stacked body of a battery element, it cannot solve the problems that arise due to repeated contacts of the stacked body of a battery element with the inside of the film casing covering the stacked body if it is applied to a battery covered by a film casing and sealed.
However, while such a technique of providing aligning sections can hold the positional integrity of the positive electrodes and the negative electrodes of the stacked body of a battery element, it cannot dissolve the problems that arise due to repeated contacts of the stacked body of a battery element with the inside of the film casing covering the stacked body if it is applied to a battery covered by a film casing and sealed.
Therefore, the object of the present invention is to provide a stacked battery which is produced by sealing a stacked body by a film casing and sealing the latter and of which performance is not deteriorated due to leakage of an electrolytic solution and intrusion of moisture from the atmosphere because of a scratch, crack or the like formed on the film casing after vibrations are repeatedly applied thereto.